For at least the last 20 years to my certain knowledge the standard 2.4GHz videosender has used a synthesised transmitter. These actually have reasonably good frequency accuracy - well within 1MHz of nominal is normal - and good-enough stability. If you listen to one on a narrowband receiver you'll hear it wandering around a few kilohertz but that's all.
Here's a close-in phase noise plot of a 1.2GHz transmitter based on identical technology, done some 20 years ago:
These transmitters all use FM, and some may have one or two sound (or maybe telemetry) subcarriers at typically
+6 and
+6.5MHz (
+ because they're subcarriers modulated onto the FM centre frequency so they appear both above and below the carrier). Here you can clearly see the two subcarriers, roughly 30dB lower in amplitude than the carrier:
So you can take it that the transmitters are going to be pretty good.
On the receive side, it very much depends on what the designers decided upon. Most use(d) the same type of synthesiser as the transmitter and, at that time, 480MHz IF.
Capture range was small, no more than 2-3MHz either side of the nominal frequency - and that was only for strong signals. Weak signals had to be more or less spot on frequency to avoid major sparklies like this example:
Putting the receiver spot-on the transmit frequency (within 0.5MHz or so) cleaned up the video significantly: this was exactly the same receiver with exactly the same
off-air test signal but tweaked very slightly to put it spot-on:
My best guess is that the ISG gear was built with very similar technologies, as that's the era it's from. More modern analogue video transmitters use more tightly integrated (and much smaller and
hotter-running) transmitters. I haven't disassembled a recent receiver to find out whether they are still analogue or are now demodulated digitally.
Why use analogue for these links when as we all know digital TV is so much more bandwidth-efficient (offering a good picture in <2MHz bandwidth compared to ~20MHz for analogue)? Probably because analogue never, ever, 'freezes' so there's no chance of thinking the camera is just pointed at a static scene when in fact it - and the fireman holding it - has plunged through a failed floor into the basement. On analogue you'd see the signal disappear and maybe get occasional snowy/sparkly frames, whereas on digital you'd (probably) just see the last good frame, or maybe a blue screen.